In a back-lighting application, one or more light emitting diodes (LEDs) provide illumination to a light guide or light pipe. A display such as a liquid crystal display (LCD) is placed over the light guide and is thereby illuminated.
White LEDs emit light that appears white to an observer and can be used for back-lighting. Red, green and blue LEDs can be used in combination to produce many colors and intensities of light for color displays as well as white light for back-lighting.
Controlling the brightness of the LED(s) is important so that there is enough illumination to make visible the information being displayed by the LCD. With the use of multiple, different-colored LEDs, controlling the brightness of the LEDs is also important in order to achieve proper color balance.
In general, a conventional light source controller employs a feedback loop that measures the voltage produced by the light received from the light source (e.g., an LED) and adjusts the light source accordingly. Conventional controllers include a sensor that converts the light from the light source into a voltage. The controller can also include a low-pass filter, a buffer/gain amplifier, and an analog-to-digital converter (ADC) to convert the measured voltage into a digital signal. The digital signal is received by a signal processor that determines whether the light source needs to be adjusted (e.g., made more or less brighter). The processor controls a pulse width modulation generator that drives the brightness of the light source.
Conventional controllers are problematic for a number of reasons. The low-pass filter, buffer/gain amplifier and ADC increase the size of the integrated circuit die, which can increase costs. Also, the low-pass filter, buffer/gain amplifier and ADC can each introduce noise into the circuit, which can effect the granularity of control.
A controller that can reduce die size and noise would be advantageous. A controller that can provide those advantages and also reduce power consumption and response time would be even more advantageous.